There have been 893 cases of sudden unexplained nocturnal death syndrome (SUNDS) from January 2001 to October 2013, up from 231 cases seen between January 1990 to December 1999.

Autopsies have revealed that these workers had no terminal illness or injury and that the workers were otherwise healthy. The report found that the young male workers suddenly had a hard time breathing while they were sleeping, though the cause for this was unknown.

Researchers at Zhongshan School of Medicine, cited by Guangzhou Daily, found that young manual laborers were most at risk. More than 90% of those that died were manual laborers and the researchers said longer hours and poor work and living conditions put them at higher risk. More than 90% of the victims were male.

Dongguan was once one of China's biggest manufacturing hubs with the city's GDP rising 19.5% annually between 2003 and 2006. But after the financial crisis, growth slumped to 5.3% in 2009. The mayor of Dongguan expects growth to pick up to 9% in 2014 as a result of investment in certain high-tech projects. More recently, the city came under the scanner as it was dubbed China's sex capital.

A recent paper on SUNDS by Chao Liu et al, published in Forensic Science International, looks at SUNDS in Southern China and related diseases from other countries. From the paper:

As an ethnic and region specific natural death, sudden unexplained nocturnal death syndrome (SUNDS) or sudden unexplained death during sleep (SUDS), is a disorder that prevails predominantly in Southeast Asia and has various synonyms in different countries such as the Philippines (bangungut), Thailand (lai-tai), Japan (pokkuri), and China (sudden manhood death syndrome).

The annual incidence of SUNDS has been reported to be as high as 43 per 100,000 people aged 20–40 years in the Philippines and 38 per 100,000 people aged 20–49 years in Thailand. In Southern China, the incidence is about 1 per 100,000 people.

These reported worldwide syndromes have an unusual clinical phenotype in common: the vast majority of victims were apparently healthy young males between 20 and 50 years old; death mostly occurred at night during sleep with symptoms of moaning, tachypnea, and abrupt tic of limbs; gross autopsy and microscopic findings showed no morphological changes to elucidate the cause of death. Since its initial description in 1917 in the Philippines, SUNDS has remained an enigma to both forensic pathologists and clinicians.

The paper examined whether SUNDS is a function of a disorder in the cardiac sodium channel, which is responsible for the function of the myocardium, a heart muscle. "Our data suggest that a majority of Chinese SUNDS may be due to other mechanisms that lead to an increased risk for sudden death during sleep," the authors write.

Zhang Yiri, an associate professor from Guangzhou City Polytechnic told China Daily that many factories pay low basic salaries but pay much better for overtime work. Zhang said this has caused many laborers to work extra hours to make more money but also puts them at higher risk.